Astronomy & Astrophysics manuscript no. aanda_ammonium-salts ©ESO 2021 April 22, 2021 Ammonia snow-lines and ammonium salts desorption ? F. Kruczkiewicz1; 2; 3, J. Vitorino2, E. Congiu2, P. Theulé1, and F. Dulieu2 1 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France e-mail: [email protected] 2 CY Cergy Paris Université, Observatoire de Paris, PSL University, Sorbonne Université, CNRS, LERMA, F-95000, Cergy, France 3 Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, Garching, 85748, Germany Received –; accepted – ABSTRACT Context. The nitrogen reservoir in planetary systems is a long standing problem. Part of the N-bearing molecules is probably incor- porated into the ice bulk during the cold phases of the stellar evolution, and may be gradually released into the gas phase when the ice is heated, such as in active comets. The chemical nature of the N-reservoir should greatly influence how, when and in what form N returns to the gas phase, or is incorporated into the refractory material forming planetary bodies. Aims. We present the study the thermal desorption of two ammonium salts: ammonium formate and ammonium acetate from a gold surface and from a water ice substrate. Methods. Temperature-programmed desorption experiments and Fourier transform infrared reflection spectroscopy were conducted to investigate the desorption behavior of ammonium salts. Results. Ammonium salts are semi-volatile species releasing neutral species as major components upon desorption, that is ammonia and the corresponding organic acid (HCOOH and CH3COOH), at temperatures higher than the temperature of thermal desorption of water ice. Their desorption follows a first-order Wigner-Polanyi law. We find the first order kinetic parameters A = 7.7 ± 0.6 × 1015 s−1 −1 20 −1 −1 and Ebind = 68.9 ± 0.1 kJ mol for ammonium formate and A = 3.0 ± 0.4 × 10 s and Ebind = 83.0 ± 0.2 kJ mol for ammonium acetate. The presence of a water ice substrate does not influence the desorption kinetics. Ammonia molecules locked in salts desorb as neutral molecules at temperatures much higher than previously expected that are usually attributed to refractory materials. Conclusions. Ammonia snow-line has a smaller radius than the water snow-line. As a result, the NH3/H2O ratio content in solar system bodies can be a hint as to where they formed and subsequently migrated. Key words. astrochemistry – molecular processes – methods: laboratory: solid state – ISM: molecules – Protoplanetary disks – Comets: general 1. Introduction the conclusion that atomic nitrogen is the dominant reservoir of nitrogen. The abundance of nitrogen, the fifth most abundant element in Other N-bearing molecules such as NO, CN, NS and NH the Universe, has always been a matter of debate (Maiolino & + Mannucci 2019), both in the extra-galactic interstellar medium diatomic radicals, or NH3, HCN, HNCO, HNC and N2H are (ISM) and in the local ISM, because nitrogen has both a primary observed with variable abundances in different sources (Omont 2007), NH3 being the most abundant of the gas-phase carrier at a and a secondary nucleosynthetic contribution (Vila-Costas & −8 Edmunds 1993), and because nitrogen has a significant depletion typical abundance of few 10 with respect to molecular hydro- factor of 0.89 (Savage & Sembach 1996) indicating that a frac- gen. In pre-stellar cores it is likely that nitrogen is incorporated tion of nitrogen has been incorporated into dust grains (mainly into the dust grain icy mantles. A recent study by Punanova et al. as titanium nitride, TiN). In the solar system as well, the N abun- (2018) illustrates this specific property and demonstrates how dance is among the more uncertain elemental abundances (Lod- N-bearing molecules can map the densest parts of star-forming ders 2010); it has been estimated to be 7.24×10−5 with respect to regions such as filaments in the Taurus molecular cloud. The hydrogen (Caffau et al. 2009; Grevesse et al. 2010). Nitrogen can hyper-fine structure of ortho-NH3 (10 – 00) has been resolved for have several carriers, depending on the phase of the ISM. In the the first time in space (Caselli et al. 2017) by observing a dark molecular cloud, confirming previous estimates of dynamics and arXiv:2104.10464v1 [astro-ph.EP] 21 Apr 2021 diffuse ISM, nitrogen is mostly atomic, neutral or ionized; the column densities derived from VUV and UV absorption lines abundances. The chemical model developed underestimates the of molecular nitrogen (Knauth et al. 2004) is several orders of abundance of NH3 towards the center of the nucleus by more magnitude lower than that of atomic nitrogen (Nieva & Przy- than an order of magnitude and overestimates its abundance by billa 2012). In dense clouds, although neutral nitrogen (NI) and about two orders of magnitude in the outer regions (Hily-Blant et al. 2010). The Spitzer ice survey showed that in star-forming molecular nitrogen (N2) are not observable directly, indirect ob- + servations using diazenylium (N H+) (Maret et al. 2006) lead to regions the most abundant N-bearing ices are NH3, NH4 and 2 OCN−, with typical (but with large uncertainties) abundances of about 4×10−6, 4×10−6 and 4×10−7 with respect to hydrogen nu- ? The authors dedicate this paper in celebration of Stephan Schlem- clei, respectively (Boogert et al. 2015), HCN being undetected mer’s 60th birthday on September 7, 2020. We thank all his contribu- in interstellar ices. However, these three main N-carriers only tions to the field of Laboratory Astrophysics. account for about 10% of the overall nitrogen elemental bud- Article number, page 1 of 10 A&A proofs: manuscript no. aanda_ammonium-salts get (Öberg et al. 2011). The missing N may be partially in the is more efficient for N-bearing and O-bearing molecules rather form of N2, but the latter is almost undetectable (Boogert et al. than for C-bearing molecules (Faure et al. 2015a,b). 2015). In Young Stellar Objects and outflows, where the ice is sublimated by radiative or shock heating (Riaz et al. 2018; To make these observations meaningful, it is important to re- Tafalla et al. 2010), N-bearing molecules abundances increase trace the journey of the main N-carriers from the pre-solar nebula but still cannot account for the initial atomic abundances. In the (a cold, radiation rich environment, favoring isotopic fractiona- + solar system, observations by the Rosetta mission have shown tion) to the solar system bodies. Although NH3 and NH4 are the that for 67/P Churyumov-Gerasimenko, like for 1P/Halley be- main observed carriers in ice, they account for less than 10 % of fore, cometary ices are depleted in nitrogen (Filacchione et al. the overall nitrogen elemental budget (Öberg et al. 2011); HCN 2019). The COSIMA instrument on board measured an average or N2 are not detected in ices (Boogert et al. 2015). While NH3 is ± + N/C of 0.035 0.011, which is similar to the ratio found in the a volatile species (it desorbs before or with water), NH4 is a re- insoluble organic matter extracted from carbonaceous chondrite fractory species, which has been detected in cometary spectra af- meteorites and in most micrometeorites and interplanetary dust ter the water ice has sublimated (Altwegg et al. 2020; Poch et al. particles (Fray et al. 2017). Comparing this ratio with the N/C 2020). Actually, the incorporation of nitrogen in the (acid) solu- value of 0.29 ± 0.12 in the solar system (Lodders 2010), we ble or insoluble matter of chondrites, depends on the amount and see that almost 90% of the nitrogen is missing. Recently, one form of N-bearing molecules inherited from the proto-solar neb- of the missing reservoirs of nitrogen may have been found in ula ice (Alexander et al. 2017), and the insoluble organic matter comets, as the 3.2 µm absorption band is likely to be due to may be an important source of Earth’s volatile species. ammonium salts (Altwegg et al. 2020; Poch et al. 2020), with the spectrum of comet 67/P having the same shape of a lab- This is why reliable modeling of disk chemistry is under de- oratory spectrum of ammonium formate mixed with pyrhotite velopment (Walsh et al. 2014; Visser et al. 2018; D’Angelo et al. grains. Ammonium salts are semi-volatile materials that desorb 2019), with the gas/surface exchange, through accretion and des- at higher temperatures than the more volatile NH3 and H2O (Viti orption, being an important part of the modeling. Although such et al. 2004), which explains why they may have been missed a complete modeling is clearly out of the scope of this paper, we by the COSIMA instrument. The temperature dependence of the want to show how different treatments of the ammonia desorp- solid to gas phase abundance ratio of each one of the reservoirs tion can lead to different conclusions. is fundamental to address the overall N budget. The solid phase abundance of each carrier is more easily quantified after its sub- In its ammonium form, nitrogen is more refractory (Gálvez limation. et al. 2010) and can be stored on grains in the inner side of the snow-line, which corresponds to the water ice mantle desorption. Indeed, NH3 is central in ice chemistry (Theulé et al. 2013), as it bears the lone pair of the N atom that favors the nucle- In the form of ammonium, the amount of nitrogen that can be incorporated into the planetesimals and later into primitive plan- ophilic addition reaction with CO2 giving ammonium carbamate + – etary atmospheres depends on its desorption mechanisms and is NH4 NH2COO and carbamic acid NH2COOH (Bossa et al. 2008; Noble et al.